drm/i915/psr: Equation changed for sending start/stop on prior line

drm/i915/psr: Equation changed for sending start/stop on prior line

Equation for sending start/end SDP prior to the SU region start/end
has changed. Update used formula.

Bspec: 49274

Cc: Mika Kahola <mika.kahola@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220905102355.176622-2-jouni.hogander@intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>